Help understanding overloading of Atomics within Block

Question

Usually h @@@ {f[1, 2], f[3, 4]} === {h[1, 2], h[3, 4]} but this is not the case when f is Complex: h @@@ {1 + 2I, 3 + 4I } === {1 + 2 I, 3 + 4 I} Since Complexis an atomic and as documentation for Apply states: Applying to atomic objects that do not have subparts effectively does nothing

Using Block to replace Complex with complex gives result as expected for non-atomic case:

Block[{Complex = complex},
 List @@@ {Complex[1, 2], Complex[3, 4]}
 ]
(* {{1, 2}, {3, 4}} *)

But then how come replacing Complex with List while not Apply-ing does not give the same result?

Block[{Complex = List},
 {Complex[1, 2], Complex[3, 4]}
 ]
(* {Complex[1, 2], Complex[3, 4]} *)

As it would have for a non-atomic head:

Block[{f = List},
 {f[1, 2], f[3, 4]}
 ]
(* {{1, 2}, {3, 4}} *)

Answer 1

This is hardly a complete answer but I suspect this is the result of special handling of the symbol Complex, much as there is special handling of packed arrays.

Remember that Block only affects things that evaluate, e.g. Block[{a = 1}, Hold[a, b, c]] returns Hold[a, b, c]. I believe that Complex may be passed over when it comes to evaluation. Consider this example outside of Block:

Unprotect[Complex];
Complex = ff;
Complex[1, 2]
Quit[]

1 + 2 I

The head Complex is never evaluated to ff here. Interestingly, with a delayed definition it is:

Unprotect[Complex];
Complex := ff
Complex[1, 2]
Quit[]

ff[1, 2]

I cannot think of a reason within the normal evaluation process for this to be, hence my suspicion of special handling.